The superconducting magnet has been generally known as a magnet that can generate a homogenous and stable magnetic field (magnetostatic field). However, the homogeneity and stability of the magnetic field do not satisfy the precision required for high-resolution NMR measurement with no change. As known in Non-Patent Document 1, in the high-resolution NMR measurement, there is required that a turbulence of the magnetostatic field is 0.01 ppm or lower within a space of 1 cm3 as a typical example, although depending on a need for measurement precision. Also, there is required that a change in the magnetostatic field with time is 0.01 ppm or lower per one hour. As a unit representative of a magnitude of the magnetic field, a ratio (ppm=1/1000000) of that magnitude to a magnitude of the magnetostatic field developed by the superconducting magnet is frequently used.
For that reason, in the superconducting magnet for the NMR, as known in Patent Document 1, the magnetic field intensity and the magnetic field homogeneity are corrected by the aid of a shim coil integral with the superconducting magnet. Setting its operating conditions to the shim coil is called “setting of the shim”, and various parameters set in the shim are called “terms of the shim”. The terms of the shim includes a Z0 (correction of a magnetic field value in parallel to a magnetostatic field), a Z1 (linear function correction of a magnetic gradient in parallel to the magnetostatic field), an X1 (linear function correction of the magnetic gradient in a direction perpendicular to the magnetostatic field), and a Y1 (linear function correction of the magnetic gradient in a direction perpendicular to the magnetostatic field and the X1) in correspondence with the magnetic field value to be corrected, a direction of the magnetic gradient, or a function form. That is, the term of the shim represents the correction magnetic field in a measurement space by its direction (X, Y, Z, XZ, YZ, . . . ), and the order (0, 1, 2, . . . ) of approximate in that direction.
The number and type of terms of the settable shim are determined depending on a device to be used. Also, in general, the set value of the shim means an array (Z0, Z1, Z2, Z3, X1, Y1, XZ, YZ, . . . ) in which the terms of the settable shim are combined together.
In this way, in the superconducting magnet for the NMR, an absolute value (term of zero order) of the magnetic field and various unhomogeneity (term of first order or higher) are generally corrected by the aid of the shim coil. To control the stability and homogeneity of the magnetic field by the shim coil is called “shimming”.
The magnetic field formed by the shim coil interacts with a superconducting coil, and the magnetic field in the measurement space intricately fluctuates depending on time in correspondence with a history of values of the magnetic field generated by the shim coil. For example, when the set value of the shim is changed with a change in a probe, a long time of one week or longer may be required until the magnetic field of the superconducting magnet is stabilized.
This phenomenon is remarkably observed in the superconducting magnets for a high-field NMR, particularly, the superconducting magnet for a solid-state NMR (1H-930 MHz=21.8T) that generates the world-class magnetic field which is developed and used by us among those superconducting magnets, and adversely affects the NMR measurement. FIG. 1 illustrates results obtained by measuring, by the NMR, a change in the magnetic field in the magnetic field space with time when a step functional set value is set to the term of the Z0 of the shim at a time (t1), and a square wave value is set thereto at a time (t2) in fact in the superconducting magnet. In the figure, a solid line represents a measured value, the axis of ordinate represents a value of the magnetic field, and the axis of abscissa represents time. The figure shows an appearance that the values of the magnetic field to be originally horizontal between the times (t1) and (t2) largely fluctuate depending on time.
In the NMR measurement, since the probe is changed for each of measurement nuclides, the set value of the shim is also intermittently changed together with that change-state. The change in the probe is frequently conducted, particularly, in the solid-state NMR measurement. That is, there frequently occurs a situation in which new setting is conducted on the shim before a previous magnetic field fluctuation is reduced and disappears, and the magnetic field fluctuation associated with this new setting is overlapped on the previous magnetic field fluctuation.
It is assumed that a principle for generating the magnetic field fluctuation occurs because it takes time for a magnetic flux generated by the shim coil to migrate within the superconductor of the magnet while repeating pinning operation and hopping operation. When a non-equilibrium state of the distribution of the magnetic flux is reduced into an equilibrium state, the migration of the magnetic flux is terminated, and the magnetic field is stabilized. The magnitude of this effect depends on the property of the superconductor used, and is more remarkable as the magnetic field of the magnet is stronger. However, in principle, it is conceivable that the same effects occur to various degrees in the superconducting magnet that generates the intense magnetic field by the aid of type II superconductor. It is expected that a countermeasure against this phenomenon becomes an essential technology as the magnetic field of the NMR becomes further stronger.
In a solution NMR measurement in which a measurement target is a solution sample, there is generally used a technique called “NMR magnetic field lock” in which deuterated solvent such as deuterated chloroform is used as solvent, and setting of the shim is controlled by the aid of the NMR signal of deuterium, and the above magnetic field fluctuation can be compensated. However, in a solid-state NMR measurement in which a measurement target is solid, this technique is not used because the sample contains no solvent. In this way, the present invention is mainly intended for the superconducting magnet used for the solid high-resolution NMR measurement. Even in the solution NMR measurement, the present invention and the NMR magnetic field lock are used together whereby a load on an NMR magnetic field lock function is reduced, and higher-precision measurement is realized. Also, as compensation of the terms other than the Z0, the present invention provides the most effective compensating means.
In the present specification, the contents of the present invention are mainly described in line with the solid high-resolution NMR measurement, but the application of the present invention is not limited to this case.
The present invention can be applied to a system that enables shimming using the shim coil for a purpose requiring stabilization of the magnetic field of the superconducting magnet, as represented by a magnetic resonance imaging (MRI). In the MRI, various magnetic field modulations are conducted by a modulation coil, and an influence of application of the modulated magnetic field of various arbitrary waveforms on the superconducting magnet can be compensated by the present invention.
Also, in the solution NMR measurement, when an experiment (gradient shim method) in which a gradient magnetic field is applied by the aid of a gradient magnetic field shim coil is conducted, an influence of the application of the intermittent gradient magnetic field on the superconducting magnet can be compensated by application of the present invention.